T ~7 au or standards

Library, n.w. Bldg

APR < 1961

NBS MONOGRAPH 26

Development of High-Temperature

Strain Gages

U.S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
THE NATIONAL BUREAU OF STANDARDS

Functions and Activities

The functions of the National Bureau of Standards are set forth in the Act of Congress, March

3, 1901, as amended by Congress in Public Law 619, 1950. These include the development and

maintenance of the national standards of measurement and the provision of means and methods for making measurements consistent with these standards; the determination of physical constants; and

properties of materials; the development of methods and instruments for testing materials, devices,

and structures; advisory services to government agencies on scientific and technical problems; invention and development of devices to serve special needs of the Government; and the development of

standard practices, codes, and specifications. The work includes basic and applied research, development, engineering, instrumentation, testing, evaluation, calibration services, and various consultation

and information services. Research projects are also performed for other government agencies

when the work relates to and supplements the basic program of the Bureau or when the Bureau's

unique competence is required. The scope of activities is suggested by the listing of divisions and

sections on the inside of the back cover. Publications

The results of the Bureau's work take the form of either actual equipment and devices or published papers. These papers appear either in the Bureau's own series of publications or in the journals

of professional and scientific societies. The Bureau itself publishes three periodicals available from

the Government Printing Office: The Journal of Research, published in four separate sections,

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Handbooks, Miscellaneous Publications, and Technical Notes. Information on the Bureau's publications can be found in NBS Circular 460, Publications of the

National Bureau of Standards ($1.25) and its Supplement ($1.50), available from the Superintendent

of Documents, Government Printing Office, Washington 25, D.C.
UNITED STATES DEPARTMENT OF COMMERCE • Luther H. Hodges, Secretary NATIONAL BUREAU OF STANDARDS e A. V. Astin, Director

Development of High-Temperature

Strain Gages

J. W. Pitts and D. G. Moore

National Bureau of Standards Monograph 26

Issued March 17, 1961

For sale by the Superintendent of Documents, U.S. Government Printing Office Washington 25, D.C. - Price 20 cents
Contents

Page

Foreword in

1. Introduction 1

2. Investigation of properties of high-temperature

strain gage materials 1 2.1. Wires 1

a. Temperature coefficient of resist- ance 1 b. Stress-strain relations. 2

c. Gage factor 2

d. Drift 3

2.2. Ceramics 4

a. Corrosiveness of cements 4

b. Effect of curing temperature on

flexibility of cements 5

c. Other properties 5

3. Development of gages. 5

3.1. Metal-backed gages 5

3.2. Plastic-backed gage 6

3.3. Asbestos paper-backed gage 6

a. Design and fabrication 6

b. Characteristics 7

(1) Gage factor 7

(2) Effect of cement thickness on

gage factor 8

(3) Strain response at high strain

levels 9

(4) Behavior of gages attached to metals of different thermal

expansivities 9

re

Page

3. Development of gages—Continued

3.3 Asbestos paper-backed gage—Continued

b. ^.Characteristics—Continued

(5) Resistance to ground 10

(6) Effect of surface roughness on

gage response 10

(7) Effect of thermal cycling on

gage behavior 10

3.4. Unbacked wire-grid gages 10

a. Design and fabrication 11

b. Ceramic-coated gage 11

c. Characteristics 11

4. Development and evaluation of high-temperature

strain gage cements 12

4.1. Resistivity 12 I 4.2. Strength 13

4.3. Hygroscopicity 14

4.4. Minimum curing temperature 14

a. Stability of cements on slow and

rapid heating to 600° F 15

b. Theory of cement instability 16

c. Relation of minimum curing temperature to operation of gages 18

5. Concluding remarks 19

6. References 19

t
Development of High-Temperature Strain Gages

J. W. Pitts and D. G. Moore

A summary is presented of a research program aimed at the improvement of hightemperature strain gages of the electrical resistance type. Potential ceramic and metal

components were evaluated and a gage was devised that was based on these evaluations. This gage (NBS 5B) was flexible and easy to install; however, it lacked resistance stability at higher temperatures. In an attempt to minimize this deficiency, ceramic cements were

developed that showed greater electrical resistivity than had been previously observed in the range 800 to 1,800 °F; also, a technique was devised for increasing the resistance to ground by applying a fired-on ceramic coating to the grid of a specially developed unbacked

gage.A study was made of the cause of the erratic response of cemented gages that had not been preheated prior to use. There were strong indications that the erratic response was

caused mostly by the rapid decrease in resistance that accompanied structural changes in the cement.

1. Introduction

The period since "World War II has seen a

greatly expanded use of electrical resistance strain

gages for the determination of strains in aircraft structures. The gages that are most often used

for this purpose consist of an alloy wire filament on a paper or resin backing that can be bonded

to the test structure by means of an organic

adhesive. These gages give good performance at

or near room temperature, but unfortunately the

maximum temperature at which they will operate

satisfactorily is about 350 °F [l]. 2 Above this temperature, both the backing and the adhesive

deteriorate and the full amount of strain is not

transmitted to the filament "wire. It is obvious

that such gages cannot be used to measure strains

at high temperatures such as, for example, in

aircraft engines, or in aircraft and missile structures that are exposed to severe aerodynamic

heating.

A logical approach towards extending the upper

temperature limit of a resistance-type gage is to

substitute a ceramic cement for the organic resin.

The development of such a gage was first reported

by Schabtach and Fehr [2] in 1944 and later by

Kemp, Morgan, and Mansion in 1947 [3]. Gages

of this type were used successfully to measure dynamic strains at temperatures as high as 1,500 °F

and, by 1954, several designs of ceramic-bonded

gages were in various stages of development in a number of laboratories [4]. However, none of these was completely satisfactory for static strain

measurements; in addition, each of the designs re- quired a considerable amount of skill on the part

of the operator to prepare and mount the gage on

the test structure. An improved high-temperature

gage was needed, and in an attempt to fill this need an investigation was initiated in February 1954 in the Enameled Metals Laboratory at the National

Bureau of Standards. The present Monograph

constitutes a final summary report on this program.

2. Investigation of Properties of High-Temperature Strain Gage Materials

2.1. Wires

The ideal strain gage wire should have the fol- lowing properties: (1) Oxidation resistance over

the useful temperature range, (2) high electrical resistivity (3) zero temperature coefficient of re- sistance over the useful range, (4) high and constant gage factor, and (5) metallurgical stability.

Xo single alloy wire is known to have all of these

properties; therefore, the selection of a particular

wire depends on its having certain desirable prop1 The work described in this Monograph was sponsored by the Bureau of Aeronautics, Department of the Navy, Wright Air Development Center, Department of the Air Force, and partially by the Bureau of Ships, Depart- ment of the Xavy.

' Figures in brackets indicate the literature references at the end pi this Monograph.

erties relevant to a particular application; or, more

frequently, the exclusion of a wire is based on its having an intolerable deficiency. The names of some of the alloy wires that have been considered

for high-temperature strain gage use, their compositions and their published characteristics are given in table 1. Certain properties of wires that were investigated in this program are discussed in

the following sections.

a. Temperature Coefficient of Resistance

Figure 1 shows the resistance changes with temperature as determined for 1-mil diameter wires

of cold-drawn Nichrome V, heat-treated Karma,

and cold-drawn 80 Pt-20 Ir alloy, all heated in air

1
Table 1. Compositions and properties of some Une wires which have been used or considered for use in high-temperature

strain gages a

Metal or alloy Nominal composition

Resistivity

at room

temperature

Mean temperature

coefficient of resistivity Mean coefficient of linear thermal expansion

Gage

factor at room temperature

Advance b Constantan. _ Copel c Cupron d Nichromeb --. TophetC d—

Nichrome V b. Tophet Ad... Evanohmd... Karma b

Alloy D • Alchrome d... Platinum

Pt-Iridium...

Pt-Iridium...

wl percent

55 Cu,45 Ni

55 Cu,45 Ni

55 Cu,45 Ni

55 Cu,45 Ni

61 Ni,24 Fe,15 Cr

61 Ni,24 Fe,15 Cr

80 Ni,20 Cr

80 Ni,20 Cr

74 Ni,20 Cr,3 Al,3 Cu.

74 Ni,20 Cr,3 Al,3 Fe.

70 Fe,20 Cr,10 Al

79 Fe,16 Cr,5 Al

100 Pt__._

90 Pt, 10 Ir 80 Pt,20 Ir

ohms per

cir mil-ft

294 294

294

294

675

675

650

650

ppm per °F

±10

±10

±10

±10

80

70

60

60

800

1060

820

64

150 200

±10 ±10

2170

720 440

temp range, "F

68 to 212

68 to 212

68 to 212

68 to 212

68 to 930

32 to 212

68 to 930 32 to 212

32 to 212 32 to 212

68 to 1830

32 to 212

32 to 212

32 to 212 32 to 212

ppm per °F

8.3

8.3 8.3

8.3

9.5 7.5

9.5

7.3

7.8

7.4

9.1

7.5

4.9

5.0

temp range, °F

68 to 212

68 to 212 68 to 212 68 to 212

68 to 1830

68 to 212

68 to 1830 68 to 212

68 to 212

68 to 212

68 to 1830

68 to 212

68 to 212

68 to 212

2.1

2.1 2.2 2.1

2.7

2.7

2.0

2.3

2.0

2.2

4.8

5.3

6.0

a Sources of data: Manufacturers' literature and references [5], [15], [17], and [18]. These data are approximate inasmuch as most of the characteristics will vary with minor changes in composition and, particularly, with

both the thermal and mechanical histories of the wires.

b. o. a Registered trade names of the manufacturers, respectively: Driverat a rate of 10 °F per minute. These results are in

fair agreement with those reported by Kemp,

Morgan, and Manson [51.

25

20

uj 5

-5

1

1 i \ . • 1

80 Pt-20Ir^

/

>"^ NICHROME I

^

KARMA

1 1 1

1 1 1 200 400 600 800 IOOO TEMPERATURE. °F

1200

Figure 1. Resistance-temperature curves for three 1-mil diameter alloy wires. Data for the curves were obtained during the first heating at a heating rate of approximately 10 °F per minute.

Harris Co., Harrison, N.J.; Hoskins Mfg. Co., Detroit, Mich.; and Wilbur

B. Driver Co., Newark, N.J.

e Development of Armour Research Foundation of Illinois Institute of Technology, Chicago, 111.

b. Stress- Strain Relations

Because the mechanical properties of very fine

wires are usually different from those of the bulk

material, the room temperature stress-strain

relations of three different 1-mil strain gage wires were determined. The wires were stressed in

tension by dead-weight loading and the length

changes were measured with a cathetometer.

Figure 2 shows the stress-strain curves for colddrawn Nichrome V, heat-treated Karma, and colddrawn constantan. The observed 0.2 percent

offset yield strength value of 79,000 psi for colddrawn constantan compares with a reported value

of 50,000 psi for normal tensile-test bars of the

material [6]. Also, the 0.2 percent offset yield

strength of the cold-drawn Nichrome V was found

to be 240,000 psi, which is higher than the re- ported tensile strength value of 190,000 psi for the

cold-drawn bulk material. If it is assumed that

the elastic limit is approximately the same as the

proportional limit, then Nichrome V, Karma, and

constantan can be used at room temperature to measure strains of about 6,000, 2,700, and 2,400

/xin./in. respectively, without plastic deformation

of the 1-mil filaments.

c. Gage Factor The gage factor of a wire is defined as the ratio

of its percentage change in resistance to its percentage change in length.

The gage factor of Karma wire was determined

at room temperature. The method consisted of

spot welding 2-in. lengths of 1-mil wire to strips

.

of Inconel foil, then simultaneously measuring

resistance changes and length changes^while the

7
CONSTANTAN

J I

I I I— 1000 STRAIN, /iin./in.

Figure 2. Room temperature stress-strain curves for 1-mil diameter strain gage filament wires.

wire was being stretched in tension. The average

gage factor of 35 specimens was 2.22 with a standard deviation of 0.22. This average is unaccountably lower than the 2.59 obtained for Karma wire

strain gages (see section 3.3b(l)).

d. Drift In making static strain measurements, it is important that the strain gage response remain

stable under constant temperature and stress conditions. There are a number of possible causes

for gage drift, some of which are associated with

the wire only. For instance, a reduction in cross- sectional area by oxidation can cause a wire to increase in resistance 3

; also, its resistance can

change at constant elevated temperature either by

metallurgical transformations or by relief of residual stresses. In order to determine the extent

of these effects, a number of measurements were made of the resistance changes occurring in unstressed, bare wires heated in air. Figure 3 shows

the change in resistance after one hour of heating

at a series of temperatures for 1-mil wires of Nichrome V, Karma, and 80 Pt-20 Ir alloy. Although these results do not distinguish between

the various contributing factors, it is probable that

structural changes are predominant at the lower

5 Kemp [7] has calculated that a 1-mil diam wire with a gage factor of 2.0 would change 100 /iin./in. in indicated strain if a surface layer of metal only

£X10-» in. thick were removed.

O- -O KARMA

« S NICHR0ME V e e Pt-20%ir

800 900 1000 1100 1200 1300 1400 1500 1600 TEMPERATURE, °F

Figure 3. Change in elevated temperature resistance after one hour of heating plotted against temperature for unprotected 1-mil diameter wires. The Karma and 80 Pt—20 Ir alloy were heated to 1,600 °F and furnace

cooled prior to testing while the Nichrome V wire was heated to 1,900 °F and

air quenched. (No measurements were made below 800 °F).

temperatures while oxidation and/or gas diffusion

contribute to resistance increases at the higher

temperatures.

Although Karma wire was found to have a comparatively low-drift rate in the temperature

range of 75 to 800 °F, it was believed possible

to reduce it further. In all previous extended

heating tests of wires it had been noted that

drift rates decreased with time at a given temperature. Therefore, it seemed likely that if the wires were pretreated at or above the anticipated test temperatures for prolonged periods, then the re- sistances would eventually become stabilized.

A series of Karma wire specimens were heat

treated at temperatures varying from 800 to 1,800

°F and for times varying from 1 to 24 hr. After

this stabilizing treatment, drift measurements

were made for a 5-hr period at temperatures of

400, 600, and 800 °F.

It was found that preheating at 1,200 °F and

above not only increased subsequent drift rates bub was detrimental to both the high-resistance

and the low-temperature coefficients of resistance,

which were originally induced in the wire by the

manufacturer's heat treatment. However, heat

treating at either 800 or 1,000 °F was effective

in reducing drift, as indicated in the table below:

Stabilizing

400 °F 600 °F 800 °F

None _ _ 0. 06 0. 04 0. 72 24 hr at 1,000 °F 0 0 . 73 24 hr at 800 °F 0 0 . 16

Percent increase in resist- ance after 5 hr additional heating at:
As a result of these findings, Karma wire was

heat treated for at least 24 hr at 800 °F prior to use in strain gages.

2.2. Ceramics

a. Corrosiveness of Cements

The effect of ceramic cements on the resistances

of fine wires was determined by embedding

Nichrome V wires in various cements, heating

the specimens to selected temperatures, and then

measuring the resistance changes of the wires at constant temperature. Specimens were heated

for 1 hr at 800 °F, then for 1 hr at each 200 °F

interval to 1,600 °F. The results for the 1,400

°F heating are shown in figure 4. These curves show that some ceramic cements have very cor- rosive effects on wires, while others are protective.

As pointed out earlier by Moore [8] the experimental cements showing corrosive attack (S-l,

S-7, S-l 9, and N-l) all contain sodium silicate as a bonding agent. C-l and C-3 are commercial

cements.

0 10 20 30 40 50 60 TIME AT I400°F, mifl

Figure 4. Change in resistance with time at 1,400 °F for Nichrome V wires: one uncoated, one coated with A-418

fused-on ceramic coating, and six embedded in six different air-drying cements {C-l, C-3, S-l, S-7, S-19, and N-l). \* >I6

in -

Because of the inertness displayed by cement

C-l in these tests and also because of its other

desirable properties, it was used almost exclusively throughout the earlier parts of this investigation lor attaching gages. This cement is a phosphate-bonded type containing quartz and

other oxides as fillers. Corrosion of the base alloy by the wet cement

can also be a serious problem. Figure 5 shows

Figure 5. Photomicrographs showing the effects of the corrosion which occurred when strain gages were cemented

to specimens of aluminum alloy 2024-

The photos were taken normal to the plane of the cement after the top

layers of cement had been scraped away and after the strain gage wires had

been removed (leaving the troughs which appear as dark horizontal bands).

what can happen when a strain gage is attached

with a cement displaying strong acidic reaction.

The specimens photographed in figure 5 were 4
prepared by cementing wire gages to aluminum

alloy 2024 with two different cements. After

drying and curing at 600 °F, the cements were

scraped away until the wires were exposed and

could be removed. The cement in figure 5A has

a 7?H of about 0.2 and vigorous reaction with the

aluminum alloy is presumed to have occurred as evidenced by the gas pockets throughout the

structure. This same cement on a corrosionresistant base showed no such effect. The cement

used for the specimen in figure 5B was less reactive

toward the aluminum alloy. The pH of the

bonding agent used in this second cement is 2.0;

also, this cement contains an acid inhibitor.

b. Effect of Curing Temperature on Flexibility of Cements

The anticipated use oi a phosphate-bonded

cement in the construction of a flexible ceramictype gage led to a study of the effect of curing

temperature on the subsequent flexibility of a

film of the cement at room temperature.

A modified bend test was devised for the ex- periments. Ingot iron specimens, 2.5 in. long by

0.5 in. wide by 0.043 in. thick were coated on one

side with a 2-mil layer of the C-l cement. Five

specimens were cured at each of four selected

temperatures and after cooling to room temperature the specimens were bent slightly in the long

direction, so that the cement layer was on the convex side of the bend. The specimens were

placed longitudinally in a compression machine

and the bending continued until a crack appeared

in the cement layer. The radius of curvature of each specimen was determined and the average

radius of each group of five specimens was plotted

as a function of temperature. Figure 6 shows

IOO 150 2 00 2 50 CURING TEMPERATURE, °F

Figure 6. Effect of curing temperature on the radius of curvature required to cause cracking of a layer of C—l

cement when applied at a thickness of two mils to ingot iron specimens, 0.043 in. thick.

the relation between the curing temperature of

the cement and the radius of curvature of the

specimen at failure. The radius at failure in- creased linearly with the curing temperature!

The stress responsible for the cracking varies

with the distance of the outer surface of the cement

layer from the neutral axis of the specimen.

Thus, the cement layers could have been bent

through much smaller radii had the backing been

foil or thin paper rather than the 0.043 in. thick- ness of iron. Nevertheless, the results show that room temperature drying produces maximum

flexibilit}^ of the cement layer.

c. Other Properties of Ceramic Cements are Reported

in Section 4

3. Development of Gages

3.1. Metal-Backed Gages

The earliest gages designed in the initial phase

of this program were constructed on backings of

ceramic-coated metal foil. The first design consisted of fine wires wound on a thin sheet of mica

which was then sandwiched between two sheets of

ceramic-coated Inconel foil of two mils thickness. In the second design the wire was wound on

coated foil and this assembly then sandwiched

between mica sheets. The third design, which

was more successful, consisted of straight strands

of wire embedded in the ceramic coating on a

single sheet of foil. Gages of the third design were constructed in the following manner: four Nichrome wires of

1-mil diameter, spaced % in. apart, were drawn

tightly across the face of a 2-mil Inconel strip

which had been precoated with 2 mils of NBS

A-418 ceramic [9]. The wires were cemented to the coated Inconel at the ends of the strip with an

air-hardening cement. Next, crossovers were formed as shown schematically in figure 7A by

brushing on NBS coating M-60F which consists

of a mixture of a glass powder and a Cr-B-Ni

brazing alloy [10]. After firing the specimen at 1,925 °F for 2 min to mature the M-60F, the ends

of the strip were sheared off to give the partly

finished gage illustrated in figure 7B. After

attaching the leads with the M-60F braze coat

as shown in figure 7C, the gage was given a final coat of A-418. The overall thickness of gages

made by this technique is about 7 mils. A trans- verse section through the gage showing one of the

wires embedded in the Cr-B-Ni alloy is shown in figure 8. Although a gage of this type is quite rugged and

therefore may find uses in certain specialized

applications, it is difficult to cement and also it is relatively inflexible because of its thickness and

construction.

578650—61 2 5
3.2. Plastic-Backed Gage

IMIIMI

ft

w ©

c 1 ( > < >

0 $

wI in.

In an effort to provide more flexibility in a gage,

the ceramic-coated foil backing was replaced by

an ethyl cellulose sheet which had been sprayed

with an air-hardening ceramic cement. Figure 9

is a detailed schematic drawing showing how the

gage is fabricated. The gage is flexible enough

to be bent around a radius of approximately onehalf inch without damage. The ethyl cellulose plastic backing sheet burns off during the first heat-up without any apparent disturbance to the

gage or its insulation. The principal deficiency of

this gage is that tbe leads are not firmly attached

and are easily pulled away from the filament.

3.3. Asbestos Paper-Backed Gage

®AIR HARDENING CEMENT

®M-60F BRAZING ALLOY — o.ooi in. diam nichrome wire — HEAVY LEAD WIRE Figure 7. Schematic drawing showing separate steps in

the preparation of a gage fabricated through use of ceramicmetal braze coat M-60F.

Figure 8. Photomicrograph of a transverse section cut through a metal-backed gage showing attachment of the 1-mil diameter Nichrome filament wire (circular area at center of micrograph) to the Cr-B-Ni alloy {upper metallic

region), and also showing the separation of this alloy from

the Inconel foil (lower metallic region) by a layer of ceramic

coating (dark region interspersed with metallic particles). Etched with Carapell'a's reagent.

a. Design and Fabrication

In an effort to make the lead attachment more

rugged and still retain the flexibility of the plasticbacked gage, another major design feature was

incorporated. A highly flexible ceramic backing

material was needed that would not decompose

during heating and to which the lead wires could

be securely fixed. A commercial asbestos paper

[1.1] was found that appeared to fulfill the requirements. This material, which comes in a standard

thickness of 3 mils, not only looks like ordinary

paper, but has about the same strength and

flexibility. The paper does not decompose on

heating to 1,500 °F although it becomes brittle. It was found that the paper can be coated with a ceramic cement by the air-spray technique and

that the resulting product is highly flexible when

the cement layer is properly applied. This composite ceramic paper became the backing material

for a new gage which was designated NBS 5B.

Essentially the 5B gage consists of two single

strands of fine wire stretched across the coated

side of the ceramic paper and spot welded to individual Inconel foil leads at one end and to a crossover strip at the other end. The construction details for a typical gage of 1-in. gage length

are illustrated schematically in figure 10. Figure

11 is a photograph of the gage. When made with

1-mil Karma wire a gage of this length has a

resistance of about 130 ohms, or, if 1-mil Nichrome

V is used, the resistance is about 110 ohms.

The gage is attached to a specimen by placing

the gage face down on a fresh layer of wet ceramic

cement. If applied directly to a surface abraded

with No. 280 grit, or finer, abrasive paper, the

chances are exceptionally good that the gage will be electrically insulated from the metal surface;

however, if 100 percent confidence against short

circuiting is essential, then the gage should be

applied over a precoated layer of cement. When

many gages are being applied and a small per- centage of malfunctions can be tolerated, it is frequently desirable to attach the gages without

6
STEP I STEP 2

Plastic Sheeting (ethyl eel lu lose) Sheared to Size os Illustrated.

STEP 4

O 0

(bo o<b

Side With Cement Layer

Placed Face-up on Lucite Block and .001" Filament

Wires Stretched Across

Holes. Wires Anchored

With Cellophane Tape at Ends.

C-l Cement Layer, .005° Thick, Applied To One

Side of Sheet and Cured

to a Flexible Adhering

Film by Drying at Room

Temperature.

STEP 5

Inconel Strips,.003" Thick, Glued to Cement Layer

and Eoch Filament Wire

Spot- Welded to Strips as Shown.Gage Sheared

to Proper Length and Excess Filament Wires

at Ends Removed.

STEP 3

0 .. 0

o o

o o o o

J^6 Diameter Holes

Drilled or Punched Thru

a Composite Sheet of

Plastic and C-l Cement.

STEP 6

TLT|

Coat of C - 1 Cement Applied Over Wires and Strips.

Inconel Leads Bent at Angle, and .OIO"Gold(or Nichrome)

Wires Brazed to Inconel. Finished Gage Can Then be Cemented to Specimen When Needed. Plustic Sheet Burns Off During Use.

SCALE

Figure 9. Schematic drawing showing various steps involved in the fabrication of a plastic-backed gage. The filament wires are completely embedded in the air-drying cement after fabrication. The plastic backing sheet burns off when the gage is heated.

a precoat, thus sacrificing the certainty of nonshorting for (1) the saving of time, (2) the advantage of a thinner gage installation, and (3)

1 a better bonded gage. When a precoat is used, there is a tendency for

the second application of the liquid bonding

agent to migrate from the freshly applied cement

into the porous, precured first layer. This loss of

bonding agent from the wet cement may result

in a poorer bond between the gage and the test specimen than is obtained when the same cement

I is applied directly to the metal.

b. Characteristics

From time to time during the course of the

program a number of properties and characteristics of developmental gages were determined more for the purpose of directing future efforts than for supplying precise evaluation data. Some

of those properties of the 5B gage which were in- vestigated are believed to have been determined

reasonably accurately while others, particularly

those involving response of the gages to strain at elevated temperatures, were evaluated by approximate methods that were considered adequate for

obtaining relative values for developmental

purposes.

The properties investigated are listed below.

In all cases, when the gage was attached to a surface, the cement used was C-l.

(1) Gage factor. The gage factors of 5B gages were determined in tension at 75 and 800 °F and

for calculated strains up to 350 judn./in. The de7
(a) 1/6 in. diameter

holes are punched

in a strip of ceramic coated asbestos paper.

N

FRONT

V

SIDE

(b) Annealed strips of 0.002 in. thick

Inconel (or other heat-resisting

alloy) are bent into the shapes shown

above.

©-<£>-

ENLARGED

SIDE CUT

(c) Upper strips (leads) are inserted in upper holes

of paper while lower end

of paper is placed in V-notch of lower strip

(cross-over). Strips are then pressed -flat

.

(d) Fine wires are spot-welded to the leads and to

the cross-over

on the face of the gage.

Figure 10. Steps in the fabrication of the 5B gage.

Figure 11. Completed 5B gage prior to cementing, showing

face of gage.

terminations were made by adding known increments of tensile loads to a heat-resisting alloy

test strip with gage attached, while reading the

resulting changes on a strain indicator. The

gage factors were computed from the instrument

readings and the known moduli of elasticity of the

test alloy at the various test temperatures. Thus,

the gage factor, K, is given by the relation

K=€iXKi/ea where e* is the indicated strain, ea the

computed strain and Kt is the gage factor setting on the instrument.

The averaged values obtained for 12 Karma

wire gages were:

Temperature Gage factor Standard

deviation

75 F° 2. 59 0. 11 800 2. 22 . 08

It is of interest that the room temperature average

value is the same as that obtained from gages of the 5B design fabricated in another laboratory

and calibrated with a Tuckerman optical strain

gage [12]. (2) Effect of cement thickness on gage factor. When the 5B gage was applied directly to a metal

surface without a cement precoat, it was found

that the gage wire was usually separated from the

surface by a layer of cement approximately 0.5 mil thick. Inasmuch as some deviations from

this separation distance might be expected when

the gages are cemented in other laboratories, tests were made to determine the effect on gage factor

of varying the cement interlayer thickness.

Gages were applied to eight stainless steel specimens with cement thicknesses ranging between 0.5 and 38.5 mils and the gage factor of each was determined at room temperature. The

8
results showed no apparent correlation between

the room-temperature gage factor and cement

thickness.

(3) Strain response at high-strain levels. Two

heat-resisting alloy specimens with 5B gages

attached were stressed in tension at 75 and 800 °F. In each case the specimen was stressed to 36,000

psi in 3,000 psi increments. The resulting indicated strain values are plotted in figure 12 as a

function of stress. The 75° stress-strain curve

is within 1 percent of the theoretical curve as calculated from the known modulus of elasticity

of the alloy and the 800° curve is within 7 percent

of the theoretical curve. Figure 12 shows that

the 5B gage is capable of giving strain indications

up to 1,270 juin./in. with a high degree of linearity.

For more accurate evaluations of response at high-strain levels a set of five randomly selected

gages were strained to gage failure at room temperature while being calibrated with a Tuckerman

optical strain gage. The strain errors are plotted

as a function of the resistance-gage readings in

figure 13. It can be seen that the maximum error

at an indicated strain of 4000 p.in./m. was less than 10 percent, therefore all gages endured true

strains of at least 3600 /nim/in. At indicated

strains of 3000 /xin./in. the maximum error was

less than 5 percent.

(4) Behavior of gages attached to metals of

different thermal expansivities. The behavior of 5B gages on three metals having widely different

• •oefficients of thermal expansion was investigated.

The three metals selected and their reported

expansivities over the range 70 to 600 °F are:

Metal Coefficient of linear ther- mal expansion

Aluminum allov 2024, 13. 7X 10-o per °F

Inconel . 7. 5 Molybdenum 3. 2

The gages were attached to small sheet speci- mens with C-l cement and, after curing for 2 hr

at 230 °F, were inserted in a furnace at 600 °F.

Changes in gage resistance were recorded for a

period of 90 min, after which the specimens were removed and cooled to room temperature.

Figure 14 is a graph of the resulting data. Each

point is the average of from 4 to 6 determinations.

The effect of the expansion of the base metal as the specimen heats from room temperature to 600 °F is obvious. The drift behavior at temperature was of a comparable magnitude for all three metals: 150, 170, and 100 Aiin./in. for inolybdenum, Inconel, and the aluminum alloy,

respectively. On cooling to room temperature,

all of the gages exhibited resistance values considerably higher than the initial value. This

zero shift is probably associated with the behavior

of the cement and will be discussed in section 4.4b.

200 4 00 600 800 1000 1200 INDICATED STRAIN, /i in. /in.

Figure 12. St7-ess-strain curves obtained with 5B gages cemented to heat-resisting alloy test strips with C-l

cement. The averaged values of gage factor (2.5S to 75 °F and 2.22 at 800 °F) which

were previously determined for the 5B gage at very low strains were used

to obtain these data.

0 2000 4000 6000

INDICATED STRAIN , fl in / in Figure 13. The errors in response of five NBS 5B re- sistance-type strain gages tested to high strain levels at room temperature.

True strains were obtained with a Tuckerman optical strain gage, from which the errors of the resistance gages were computed.

4000

5000

: 2000

b 1000

:

: 0

i

! -looo

| -2000

-3000

-4000

1 1 l l l 1 i i

ALUMINUM ALLOY (Exp 13.7 x I0 6 /°F)

l 1

?—

1

<\ \

\

1 \

1 \

1 . 1 s

\

1

1

1 -

1

INCONEL (Exp 7.5 x 10 6 /°F)

\

1 J>—

/ /

/ f—\

\ - \ / /

\ \

/

/

\ /

V

\ MOLYBDENUM (Exp 3.2 x 10 6 /°F

/ - - _r/ ^-

HEATING SOAKING COOLING

R.T.TO

'

600 "F

i

SPECIMENS HELD AT 600 'F

1 1 1 I.. X 1 >

' r 600 f ~*

TO 81

' 1

10 20 30 40 50 60 70 TIME, min

Figure 14. Indicated strain from gages of the 5B design

when attached with C-l cement to metals of widely differing thermal expansivities and preheated to 230 °F prior to testing. Each specimen with gage attached was heated rapidly by inserting into a

furnace at 600 °F. After 80 min. at temperature the specimens were re- moved from furnace and cooled in air. Each curve is the average of from 4 to 6 determinations.

9
(5) Resistance to ground. The electrical resist- ance of the cement or other insulating material

separating the strain gage circuit from the substrate is hereinafter referred to as the "resistance

to ground" or the "leakage resistance." The

resistance to ground is principaUy a function of

both the resistivity of the cement and the thickness

of the cement interlayer, but is also dependent on

the gage design. Inasmuch as the 5B gage has

a comparatively large surface area and is normally

applied without a precoat of cement on the

specimen, some measurements were made of its resistance to ground at a series of temperatures

up to 1,300° F. Gages were cemented to speci- mens with C-l cement, which was cured at 230 °F

for 2 hr. The assemblies were then heated and

held at the desired temperature until the resistance

to ground reached a constant value. The data

for two gages are shown in figure 15.

(6) Effect of surface roughness on gage response. Three groups consisting of three specimens each

of type 321 stainless steel were prepared with

widely differing degrees of surface roughness. The

specimens of one group were sandblasted with a 60 mesh sand, those of a second group were

abraded with No. 280 grit silicon carbide abrasive

paper, and the specimens of a third group were

electropolished. Also, two groups of aluminum

alloy 2024 specimens were sandblasted and

abraded as described above. Gages of the 5B

design were attached to the metal specimens with

C-l cement which, after air drying, was cured

at 220 °F. Each specimen was heat cycled

from room temperature to 600 °F a total of six times and the resistance change for each heating

cycle was measured. Uncemented gages were

subjected to the same cycling treatment and by

subtracting the resistance changes of the un- cemented gages from those of the cemented gages, values were obtained that were independent of the

temperature coefficient of resistance; i.e., each

value represents the thermal strain in the filament

that resulted from differential changes in dimensions of the wire, of the metal specimen, and of the

cement used for attaching the gage. Figure 16

is a plot of the resulting data. The small changes

of indicated strain related to differences in surface

preparation of stainless steel are not considered

significant. In the case of the aluminum allov,

the thermal strains of gages on the mildly abraded

specimens were less than would be anticipated

from thermal expansion differences only; however

gages on sandblasted specimens showed strains

unaccountably greater than would be expected.

The implication from these data is that surface

roughness affects the degree of strain transmission

at high-strain levels, but that at low-strain levels

sufficient bond between the cement and the alloy

is achieved even when the specimen surface has a high polish.

(7) Effect of thermal cycling on gage behavior.

Figure 16 shows that at low-strain levels gage

response was stable after the first cycle. The

TEMPERATURE, °F

Figure 15. Resistance to ground measurements for two SB gages attached to an Inconel specimen with C-l

cement.

4000

IO00

0

i i

i i

i

* "A 1

SANDBLASTED

,MILDLY ABRADED

o O—: O O ALUMINUM ALLOY 2024

( EXP 13.7 x I0"6

/

° F )

ELECTROPOLISHED / MILDLY ABRADED ^

/ / SANDBLASTED

§

STAINLESS STEEL AISI 321 (EXP 9.2 x I0" 6

/ • F )

i i i i i 2 3 4 5 6 CYCLE NUMBER

Figure 16. Thermal strains transmitted to SB gages when

alloy specimens with gages attached were heated from room temperature to 600 °F.

Cycle numbers refer to successive heatings.

behavior on the first cycle is believed to be due to improper curing of the cement and is discussed

in a later section (4.4).

3.4. Unbacked Wire-Grid Gages

The need for better electrical insulation at the

higher temperatures led to the development of

the unbacked wire-grid gage (design No. 6) and

eventually to the ceramic-coated gage (No. 6A).

As is well known, the electrical resistivity of ceramics decreases with increasing temperature.

10
At operating temperatures of about 1,000 °F and

higher most of the known ceramic strain gage

cements become so electrically conducting as to seriously affect the response of a strain gage. It appeared that the most promising solution to the

problem was to provide electrical insulation independent of the bonding cement. This could be

done by precoating the wire with an impervious

highly insulative ceramic material. However,

none of the available gages was suitable for applying a fused-on coating; therefore, a special type of

gage needed to be developed.

a. Design and Fabrication

A gage was needed which would maintain its grid configuration free of any backing or embedding material. As a result, a technique was

developed whereby the gage wire is given a permanent set by stretch-forming. The procedure consists of winding the gage wire, with leads attached, on pins inserted in holes in a two-piece variablelength jig, then stretching the grid in the axial

direction by elongating the jig with a micrometer

screw attachment. An alternate variation of

stiffening the grid is to heat the wire by passing an electric current through it and, while the grid

is elongated due to thermal expansion, the slack

is taken out by extending the jig. Then, as the

wire is allowed to cool, it becomes plastically

deformed as a result of restrained thermal contraction. After taping the leads to a plastic

carrier with adhesive tape, the gage may then be

removed from the jig and handled by its leads

without altering the grid shape. The gage is now ready to be ceramic coated, or if desired, it can be used uncoated.

b. Ceramic-Coated Gage

Very fine wires are not amenable to being coated

by the usual ceramic coating techniques in which

the article is either dipped in a slip (finely divided

coating material suspended in a liquid) or the

slip is sprayed onto the article, followed in both

cases by high-temperature fusion. An alternate

method of applying the slip is by electrophoresis. In this process, finely divided particles which are

electrically charged with respect to a dispersing

medium, are deposited on an electrode under the

influence of an electrical potential. This method

is particularly suitable for coating strain gages

because of (1) the uniformity with which the

coating can be applied, (2) the absence of mechanical force on fragde parts during deposition, and

(3) optional codeposition of two or more materials

simultaneously.

The technique was adapted to coating of the

unbacked gage by immersing the wire grid and

a portion of the gage leads in a dielectric liquid

containing suspended particles of a pulverized

ceramic material. The gage was made the elec- trode which received the charged particles when

a potential was applied. A number of coating

materials were tried, but the most successful

coating was a commercial high-temperature ce- ramic frit having the following computed oxide

composition:

Oxide Wt. percent Oxide Wt. percent

Si02 48. 0 A12 03 3. 6 BaO 18. 2 Na2 0 2. 6 Ti02 8. 4 K2 0 i. i B2 03 7. 9 CuO i. i ZrO 4. 7 F2 0. 1 ZnO 4. 3

The frit was ball-milled dry for 24 hr, then the —325 mesh fraction was mixed with isopropyl

alcohol in the ratio of 200 g/liter. The gage

(anode) was immersed in the suspension 1 in. from the cathode (stainless steel) and a 200 v

potential was applied to the circuit for 0.2 sec. The resulting deposit of particles was dried and

fused to a vitreous coating of approximately 1-mil

thickness by firing for 4 min at 1,500 °F. Fig- ure 17 is a photograph of a 120-ohm ceramiccoated gage. After bending the leads out of the plane of the

grid, the gage is ready for use. The bend is made

about one-eighth of an inch from the junction

with the wire so that a portion of the leads will be embedded in cement. The gage can be applied

to a test surface by laying the grid down on a

thin layer of wet cement, no precoat being

required.

c. Characteristics

These gages have been made in lengths of 5/n

to % in. using 1-mil wire. Longer gage lengths

tend to require a support for the grid, while shorter

lengths require too many loops to achieve the

Figure 17. Photograph of a ceramic-coated unbacked

gage (design no. 6A)

.

The gage length is one-half Inch and the resistance of the gage is 120 ohms.

11
necessary resistance. Grids with five loops (six

legs) have been made, but the three-loop gage

(fig. 17) is preferable.

These gages suffer from the fact that they can

be used only on plane surfaces; also the ceramiccoated gage is difficult to make and is very fragile.

No accurate evaluations have been made of the

gage; however, a few trials have shown that the

gage can be cemented easily to a test strip and

when the strip is pulled in tension the gage response

is in the approximately correct strain range.

Figure 18 shows the strain response of an unbacked, uncoated gage (design No. 6) cemented

to a test strip with NBS cement No. X-142

and strained in tension at 600 °F. The specimen was strained to 470 /iin./in. and relaxed

to 190, strained again to 470 and relaxed to 190,

then strained to 1130 and relaxed to zero, and

finally strained again to 1130 and relaxed to zero. For clarity in reading the graph, the beginning of

each cycle after the first has been shifted to the

right by 100 units. The actual strain was computed from the stress applied to the specimen and

its known modulus of elasticity at the test temperature. The gage response was measured with a

strain indicator. The instrument was zeroed only

once, at the beginning of the test; thus it can be

seen that the total zero shift was only 25 /xih./in. Another gage of the same type, but bonded with

cement C-l, was tested and gave comparable

results on the fourth cycle; however, the pre- liminary cycles were considerably more erratic. One of the effects of applying a fused ceramic

coating to the gage wire was demonstrated by

measuring the drift of coated and uncoated wires

heated in air. One mil strands of Nichrome V

and Armour Research Foundation's Alloy D, some

uncoated and some coated with a ceramic frit were heated to 1,000, 1,200, and 1,400 °F and held

for 2 hr while drift measurements were made.

Figure 19 shows the results. The negative drift (decrease in resistance) of Nichrome V at 1,000 °F

is apparently due to a time-temperature-dependent

structural change taking place in the alloy; the

positive drift of the uncoated wires is attributed to surface oxidation, the effect of which completely

masks the internal effect just mentioned, and

results in a net increase in resistance. ARF

Alloy D showed negligible drift at 1,200 and 1,400

°F. whether coated or uncoated.

Figure 18. Four strain cycles for an unbacked gage

(design no. 6) bonded with NBS X-lJf.2 cement and tested in tension at 600 °F. The beginning point of the second cycle is the end point of the first cycle, except shifted 100 units to the right. Likewise for the third and fourth cycles.

•

time , min

Figure 19. Drift

i *

of coated

- and uncoated Nichrome V and

A

Armour Research Foundation Alloy D wires.

The beneficial value of a ceramic coating as an

electrical insulator is discussed in section 4.4.

4. Development and Evaluation of High-Temperature Strain Gage Cements

4.1. Resistivity

A ceramic cement for attaching a high-temperature strain gage to a metal surface should provide

a suitable bond between the strain-sensitive ele- ment and the underlying metal, and in addition,

it should have a high-electrical resistance. Many

of the cements in current use have been found to

provide an adequate bond, but unfortunately they

all show appreciable electrical leakage at elevated

temperatures. Such leakage affects gage response, and therefore limits the usefulness of the gage. In an attempt to overcome this deficiency, an

investigation was made of the resistance-temperature characteristics of several promising cement

materials with the hope that by blending high- j

12
purity oxides with selected bonding media, cements

having very high resistivities could be developed.

A number of ceramic materials, which a review

of the literature indicated had high resistances, were obtained as powders in the highest purity

available. These ceramics included alumina,

beryllia, magnesia, thoria, silica, and mica. The

bonding agents investigated were orthophosphoric

acid, monoaluminum phosphate, calcium aluminate, potassium silicate, silica gel, and clay. Experimental cements were prepared by mixing in

various proportions each of the six bonding agents

with one or more of the different fillers and adding

water, where necessary, to produce the desired

consistency. A rapid qualitative screening test was employed to eliminate those compositions that

showed insufficient adhesiveness, unsatisfactory

hardness, or excessive shrinkage after curing at 600 °F. Then the more promising compositions were evaluated for leakage resistance at elevated

temperatures.

Measurements were made on cement specimens

placed between hollow cylindrical electrodes as shown in figure 20. Specimen thickness was controlled by inserting 0.008 in. thick separators between the tabs of the top and bottom electrodes.

These separators were removed as soon as the

cement reached a "leather-hard" condition after which the specimens were dried overnight at room

temperature, and then cured for 1 hr at 600 °F.

Resistance measurements were made at 25 °F

intervals from room temperature to 1,800 °F with a 1.5 v d-c ohmmeter. The heating rate was

15 °F/min. In addition, a few specimens were

tested through a second heating and cooling cycle. All showed increases in resistance at comparable

temperatures during the second cycle. Resistivities were computed from the measured resistances

and the known specimen dimensions. (Further

details of the test apparatus and procedure are given in ref [13].)

Figure 20. Photograph of hollow cylindrical electrodes used for measuring the resistivities of cements. Shown left to right are: an inverted top electrode, a bottom electrode, and

a set of electrodes in test alinement with a cement layer sandwiched between

them. Three legs on each electrode are used for obtaining a cement layer of uniform thickness; the legs are separated by an air gap during test.

Several of the new compositions had resistivities in the 1,200 to 1,800 °F temperature range higher

than any of the known commercial cements, yet

they were all susceptible to low resistivities in the

intermediate temperature range during the first heating schedule. Such behavior is believed to

result from the fact that the cements were not

completely "cured", although they were preheated

for 1 hr at 600 °F. This intermediate temperature

effect is not repeated on second or subsequent heating cycles, but unfortunately, the primary interest

in a strain gage cement for many applications is its behavior on the first heating.

A composition was developed that minimizes the

aforementioned effect. In addition, it maintains

the highest resistivity in the 800 to 1,800 °F range

of any of the experimental or commercial cements

tested. In table 2 the resistivity of this cement,

designated X-142, may be compared with the

resistivities of all commercial cements measured

during the course of this program. 4 The batch composition of strain gage cement

X-142 follows:

Ingredient Parts by

weight

Alumina a _ ... 100

Silica (crystalline) b

. _ 100 Chromic anhydride (CrOa) 2. 5

Colloidal silica solution °_ 200 Orthophosphoric acid (85.7%, ACS)-- 30

a 0.3 m approximate particle size.

t> —325 mesh.

0 Ludox, S.M. grade (available from E. I. duPont de Nemours, Grasselli Chemicals Dept., Wilmington, Del.).

Inasmuch as ceramic cements are polarizable

ionic conductors, their resistivities are dependent

not only on temperature, but also on the magnitude and time of passage of a direct current through them. For this reason the resistivities as herein reported cannot be considered as absolute values, but rather as relative values applicable

only to these particular test conditions. However, the fact that these values do give fairly

rehable indications of the leakage resistance of a cement in a normal strain gage application is illustrated in figure 21, which shows good qualitative agreement between the changes in resistivity

values of two cements and the changes in resist- ance to ground measurements made on two 5B

gages cemented to specimens with each of these

cements.

4.2. Strength

Upon completion of the resistance measure- ments of each specimen, the room temperature

bond strength of the cement was determined by

* Only those cements which developed a measurable bond strength were

included in resistivity comparisons, inasmuch as the extent of cementelectrode contact of the others was questionable. Intimate contact over the

entire electrode area is obviously essential to good resistance measurements.

13
400 600 800 1000 1200 1400 1600 1800 TEMPERATURE, ° F

Figure 21. The resistivity and resistance to ground of cement X-142 compared with those of commercial cement

C-l.

The average cement thickness of the resistance to ground specimens was

1.6 mils for X-142 and 3.7 mils for C-l. (Dashed lines indicate uncertain

region.) means of a cantilever-type bending test. From

the amount of force required to break the elec- trodes (see fig. 20) apart, and the dimensions of

the system, the approximate critical bending stress was calculated. This procedure gave a value for the cohesive strength if the break occurred in the

cement; otherwise the value obtained was a measure of the bond strength between the metal

and the cement. The averaged strength values

of two or more specimens per cement are shown

in table 2.

After air drying and even after heating to intermediate temperatures, cement X-142 is quite

soft compared to many other cements; yet, after

heating to 1,800 °F, it develops reasonably good

bond strength (270 psi). It is not known what

effects the variations in hardness and strength of a cement have on strain transmission, although it would seem that good strength is essential to withstand high-strain levels. The fact that cement

X-142 is capable of transmitting strains of intermediate level was shown in figure 18.

4.3. Hygroscopicity

The moisture resistance of several strain gage

cements at room temperature was determined by

measuring the weight gain of specimens exposed

to a relative humidity of 93 percent for 24 hr. The specimens used for the test consisted of layers

of cement about 0.01 in. thick spread out over an

area of one-half square inch on a metal backing.

Curing consisted of air drying for 16 hr and heating to 220 °F for 1 hr. The 93 percent relative

humidity was produced by a saturated solution of

potassium nitrate in a closed container maintained

at a temperature of 73 ±4 °F (14). The results are shown in table 2. It can be seen that cements C-6 and C-l 3, which are bonded with a silicone resin absorbed

the least amount of moisture (0.6 and 0.7 percent

respectively). The silica-containing phosphate

types of cement sorbed from 5.8 percent for X-142

to 24.9 percent for C-l and C-4. The effect of

moisture on the behavior of cements is discussed

in the next section.

4.4. Minimum Curing Temperature

Because of the interest in low-temperature

setting cements, a study was made of the reactions

taking place as a strain gage cement is cured, and

the effects of those reactions on gage response.

Table 2. Some measured properties of strain gage cements

Cement

Resistivity megohm-cm

Bending

strength •> Hygroscopicity ° 400 °F 600 °F 800 C F 1,000 °F 1,200 °F 1,400 °F 1,600 °F 1,800 °F

psi percent NBS X-142 >1000 500 >1000 >1000

90 >1000 530.0 29.2 6.8 270 ' 5.

8

NBS IC-109-. >1000 >1000 440 28 7.6 2.4 1.4 608 20.0 C-l >1000 >1000 192 71 23 4.8 0.9 0.4 >900 24.9 C-2 >1000 >1000 640 47 7 2.9 5.4 6.6 384 11.0 C-3 >1000 >1000 >1000 >1000 >1000 275. 0 21.0 1.8 0 9.3 C-4 >1000 85 2 1 2 2.0 2.0 0.4 171 24.9 C-5..._ _ _._ >1000 >1000 >1000 >1000 150 62.0 8.8 0.7 0 19.5 C-6 >1000 >1000 >1000 >1000 128 41.0 9.7 3.0 0 0.6 C-7- >1000 170 2 1 3 3.0 1.7 1. 5 0 18.8 C-9 >1000 >1000 500 6C0 300 49. 5 8.2 2.2 0 20.0 C-10 >1000 >1000 870 160 36 5.9 1.7 0.6 444 20.6 C-ll >1000 >1000 900 67 5 0.9 0.4 0.4 344 8.1 C-12 >1000 >1000 400 90 11 1.5 0.8 0.5 176 16.3 C-13 645 112 114 50 3 <0.1 <0.1 <0.1 >900 0.7 C-14 >1000 >1000 >1000 >1000 520 160.0 25.9 3.7 0 3. 6 C-15 >1000 >1000 >1000 400 120 63.0 17.0 2.3 0 1.9

» Measured at a heating rate of 15 °F per minute for specimens that had c Increase in weight of cements as a result of water pickup when specimens, been cured for 1 hr at 600 °F prior to testing. which had been preheated to 220 °F, were exposed for 24 hrs to an atmosphere

b Measured at room temperature on specimens that had been previously of 93 percent relative humidity at 73 °F. heated to 1,800 °F for testing of resistivity.

14
The two goals of the study were to determine (1) the cause of cement instability and (2) the minimum thermal pretreatment required to make a cemented strain gage reproducible on repeated

cycles from room temperature to selected elevated

temperatures.

Briefly, the experimental procedure consisted of cementing strain gages to stainless steel specimens,

preheating the specimens to various temperatures,

and then measuring the resistance changes of the

gages during repeated temperature cycles. Speci- mens were cycled between 75 and 600° F at a rate

of either 15° F/min or 150° F/min. In addition, a few specimens were cycled between 75 and

800° F at a rate of 150° F/min.

The criterion that was used to decide whether or not a cement was properly cured was to determine

the degree of repeatability between the temperature-resistance curves of the first and second

cycles. The maximum permissible resistance

spread was arbitrarily set at 300 /johms/ohm

which for a strain gage having a gage factor of 2.4 would be equivalent to an apparent strain of

125 juin./in. For various gage installations, however, the error might be larger or smaller than

125 /iin./in. depending on such factors as type of

gage and thickness of the cement precoat.

A total of 12 different cements were tested and numerous determinations were made for each

cement. First, a stainless steel specimen (2.0 in. by 0.75 in. by 0.043 in.) was uniformly precoated

with a 3-mil layer of a cement. Then the gage, which consisted of a 1-in. long, 70 ohm straight

strand of Karma wire spot welded to Inconel

ribbon leads, was cemented over the precoat.

After air drying for 16 hr, the specimen was pre- heated for 2 hr at 220 °F. Next, the specimen was

temperature-cycled twice or more while the

apparent gage resistance was measured at 25° F

intervals. This procedure was repeated for a second specimen, except that the preheat temperature was either raised or lowered as indicated by

the results of the first test. Additional specimens were then tested until the mmimum temperature was found which would result in repeatable

cycles or until it appeared that the cement would

not be suitable as a low-temperature curing type.

Several of these cements were previously found

to be hygroscopic and it was noted in preliminary

tests that high-humidity conditions had an adverse

effect on the resistance values; therefore, all data were taken from specimens tested while the room

temperature was maintained between 72 and 82° F

and the relative humidity was less than 45 percent. There appeared to be no significant moisture

pickup by the cements under these conditions.

a. Stability of Cements on Slow and Rapid Heating

to 600 °F

Figure 22 shows the temperature-resistance curves for a gage cemented with a sodium silicate type of cement and heated to 600 °F at a rate of

2800

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0 100 200 300 400 500 600 TEMPERATURE °F Figure 22. Heating and cooling curves at 15 °F/min for a gage cemented with a sodium silicate cement.

Specimen was preheated for 2 hr at 250 °F. The ordinate is the apparent

resistance change of the gage wire caused by leakage through the cement.

15 °F/min. The manufacturer of this cement

states that it will cure at 150 to 180 °F, but it is obvious that even after 2 hr at 250 °F the leakage

resistance of the cement lacks stability. It will also be noted that the shape of the second heating curve indicates that the cement becomes very

conductive above about 500 °F.

None of the cements tested was found to stabilize at room temperature; however, two cements,

which are believed to contain potassium silicate as the binder, gave reproducible results after pre- heating for 2 hr at only 130 °F. The curves for these cements are shown in figure 23. All of the

other cements tested, including the phosphate

types and the silicone resin types required pre- heating at 220 °F or higher to give comparable

results. Usually the first two heating curves, only, were

determined, as it was found that large variations

occurred only between the first and second cycles.

The small variations which occurred between the

second and subsequent cycles may be due mainly

to drift of the wire. Apparently, the first heating

to 600 °F completely stabilizes these cements

with respect to subsequent heatings to 600 °F.

Specimens of six of the cements were tested at a heating rate of 150 °F/min. The two potassium

silicates and one of the phosphates behaved nearly

the same as they did at the slower rate of 15 °F/

min. The other three cements required a higher

curing temperature (300 °F) to stabilize them.

15
TEMPERATURE, °

Figure 23. Heating curves at 15 ° F/min for experimental cements A-2 and A-10, preheated for 2 hr at 130 °F. Numbers on curves indicate first, second and third heatings. (Cooling

curves omitted.)

After being given the minimum preheating re- required for stable operation to 600 °F three

cement specimens were cycled from room temperature to 800 °F at 150 °F/min. None fell within

the limits established for reproducibility. One of

the potassium silicate cements caused a very large

positive zero shift after the first cycle. Examination of the specimen revealed numerous cracks in the cement and some separation of the cement from

the base. Apparently, the cement is dimensionally unstable for rapid cycling to 800 °F, after only

a 130 °F cure.

b. Theory for Cement Instability

It is postulated that one of the reasons for

cement instability after a low-temperature preheating treatment is the volatilization of chemically held water (or other constituent) when the

cement is subsequently heated to a higher temperature. The disturbance in the cement struc- ture caused by the volatilization could increase

momentarily the electrical conductivity, and thus

tend to short circuit the strain gage. This

hypothesis can best be explained by following the

temperature cycle shown graphically in figure 24. The cement in this case was a phosphate type,

dried overnight in an atmosphere of 45 percent

t I I l r

IOO 200 300 400 500 600 TEM PERATURE , °F

Figure 24. Heating and cooling curves at 15 °Flmin for a phosphate-bonded cement (C-10) after 16 hours air drying {no preheat treatment)

.

relative humidity, but given no preheat treatment.

As the specimen begins to heat at 15 °F/min, the

adsorbed and/or absorbed moisture begins to vaporize, reducing the resistance of the cement and

thereby reducing the apparent resistance of the

wire. At about 175 °F most of the moisture that

is not structurally held has been driven off and the

resistance of the cement begins to increase as the

last traces leave the structure. This increase is reflected by a sharp rise in the apparent resistance

of the wire. Next, at about 240 °F the evolution

of structural moisture begins and proceeds at a rapid rate, which is reflected by a large reduction

in resistivity of the cement. The effect of this

evolution of structurally held water reaches a maximum at about 265 °F. Finally at about 325

°F virtually all of the structurally held water has

been evolved, and the cement has achieved a stable

state. Further heating and cooling will cause no

sharp changes in its conductance. The room

temperature zero shift is probably due not only to the fact that the now thoroughly dry cement is more insulative than it was initially, but also to other causes such as structural drift of the wire,

corrosion of the wire, and shrinkage of the cement.

Figure 25 shows the behavior of four different

specimens of this same cement on the first heating

to 600 °F after various preheat treatments. It can be seen that 16 hr at 120 °F produced sufficient drying to eliminate the initial lowering of resist- ance shown in figure 24, but there remained a sharp decrease in resistance having maximum

16
TEMPERATURE, °F

Figure 25. First heating curve for each of four specimens

of a phosphate bonded cement (C-10).

The temperature and duration of precuring treatment is shown for each

curve. The three lower curves were obtained at a heating rate of 15 °F/min,

the top curve at 150 °F/min.

Figure 26. Apparent changes in wire resistance (curve A)

and resistance to ground (curve B) during the first heating

at 15 ° FI min for a cement specimen (C—12), that had been preheated for 2 hr at 220 °F.

effect at about 265 °F. The zero shift (not shown) was also still excessive. Preheating for either 16

hr at 150 °F or 2 hi' at 220 °F was sufficient to eliminate the erratic effect shown in the bottom

curve. However, when the heating rate was

increased from 15 °F/min to 150 °F/min (the top

curve in fig. 25) an inflection occurred, beginning

at about 350 °F. This behavior could mean that

a minor amount of "curing" of the cement occurs during a test in which the heating rate is slow; on fast heating, however, the small amount of

i water remaining in the structure comes out sud1 denly, thereby disrupting the structure and

lowering the resistance.

The belief that the lowered apparent resistance

of the wire is caused by leakage through the

cement was strengthened by two experiments.

First, tests were made in which changes in appar-

• ent wire resistance and resistance to ground were

i measured simultaneously during the first heating

I of each of several cemented specimens. Figure

j 26 is typical of the results. The curves are for a

! cement which had been preheated for 2 hr at 220

I °F. It will be noted that the inflections in both

curves (resistance to ground and apparent resist- ance of the wire filament) occur at approximately

the same temperature.

Despite the expected uncertainty in measuring

the resistance to ground, the apparent change in measured resistance of the wire is in fair agreement

with the change calculated by assuming that the

resistance to ground is an electrical shunt in parallel with the gage. For example, at 75 °F

the apparent increase in wire resistance after heating to 600 °F was 320 /xohms/ohm, whereas

the increase calculated from the parallel resist- ances (bare wire: 70 ohms; cement, initial: 0.3 megohms; and cement, final: 10 megohms) is 230

^ohms/ohm.

The second substantiating experiment involved

coating of the gage wire and leads with an impervi- ous porcelain enamel coating. At temperatures

up to 600 °F, such a coating should provide

adequate insulation for the gage, even though the

resistance of the cement dropped to a very low

value during the first heating. Figure 27 fllus- trates the effect of the precoating. The lower two

curves show the resistance changes of an uncoated

resistance wire that had been cemented with a phosphate cement, air-dried 16 hr, and then heated

to 600 °F at 15 °F/min. The upper curves are the first three heating curves for a similar specimen

except that in this case the gage (wire and leads)

had been precoated with a 1-mil application of a porcelain enamel ground coat prior to cementing.

The inflections present in the first curve for the

17
20,400

E

z

0

1

1

-

i i

i

J7 COATED

-

/ UNCOATED

n. m

i

i i i 100 200 300 400 500 600 TEMPERATURE, 'F

Figure 27. Resistance changes of coated and uncoated

single-wire Karma gages that were attached to stainless

steel with cement C-10 and cycled between room temperature

and 600 °F. Cement was uncured (air dried only), but coated gage was protected with

ceramic coating which was prefired on at 1,350 °F.

spot welding of a strand of "as-received" Karma

wire. This technique not only preserved the

low-temperature coefficient of resistance of the

Karma wire but also resulted in reproducible gage

response for successive cycles to 600 °F at 15

°F/min when the gage was attached with a potassium silicate cement and air dried only.

It is not known whether or not this method would

provide sufficient insulation where other cements

or other gage types are used.

Two of the cements included in these tests contain silicone resins as the binder. These

cements were found to have negative zero shifts after the first heating cycle, in contrast to the

all-ceramic cements which displayed positive zero

shifts. This could mean either the silicone

cements were more conductive after the heat cycle or that shrinkage of the cement resulted in a compressive stress on the wire. To check the former,

the resistance to ground was measured during a test of one of these cements which had been preheated

for 2 hr at 250 °F. The resistance to ground was initially greater than 10 megohms at room

temperature, dropped to 2 megohms at 600 °F,

but returned to a value greater than 10 megohms

at room temperature, thus ruling out leakage

through the cement as the cause of negative zero

shift. The stresses imposed on a wire by shrinkage

of the cement can be quite complex, but in general

they are compressive. If it is assumed that all of the stresses are compressive, then it follows that ! the axial stresses are the more effective, tending

to shorten the wire and thus decrease its resistance

rather than the radial stresses, which would reduce

the cross-sectional area of the wire and increase

its resistance.

uncoated specimen did not occur for the coated

specimen and in the latter case the change in resistance remained reasonably constant in successive cycles. This observation together with

the measurements of resistance to ground, indicate

quite strongly that much of the trouble from the

erratic response of gages that have not been

properly cured comes from electrical leakage in the cement.

A comparison of the slopes of curve 2 for both

the coated and uncoated wire in figure 27 shows

that the temperature coefficient of the Karma

wire has been increased by a factor of about 14

by the firing treatment (4 min at 1,350 °F)

required for maturing the enamel ground coat.

It is unfortunate from a practical standpoint that

this firing treatment destroys the low-temperature

coefficient of resistance of the Karma wire.

However, it was found that precoating only the

lead strips provided the necessary electrical insulation for gage stability, probably because the

surface area of the wire is small compared with

that of the leads. The Inconel leads were enamel coated, after which just enough of the

coating was removed from the lead tips to permit

c. Relation of Minimum Curing Temperature to Operation of Gages

Mention was made earlier that the magnitude of

the apparent strain errors indicated by these

results is not necessarily the same as would be

encountered in an acutal strain gage installation.

If the error is due to leakage through the cement,

the size and shape of the gage and the thickness

of the cement precoat are important factors. For instance, in this work when the gage consisted

of a 1-in. long (70 ohm) strand of wire spot welded

to ribbon leads, it was found that nearly all of the

leakage was through the cement adjacent to the

embedded portion of the leads. If it is assumed

that the leakage from the leads remains constant,

it can be shown that a higher resistance of the

sensing element would result in a greater error; but, reduction in surface area of the leads would be

expected to reduce the leakage. On the other

hand, gages of the etched-foil type would probably

show large errors because of leakage from the large

surface area to the base and leakage between the

many closely spaced legs of the gage. Of course,

all types of gages would show greater leakage

errors if the cement precoat thickness were reduced

below the 3 mils used in this work.

18
5. Concluding Remarks

The principal goal of the present investigation was to develop a resistance-type gage that would

be suitable for measuring static strains up to 1,500 °F. During the course of the program the

various properties of possible gage materials were

evaluated. In addition two new types of gages were developed and certain of their performance

characteristics were determined. Also, a number

of strain gage cements were evaluated, and finally a new cement was devised that showed superior

electrical resistivity in the range 800 to 1,800 °F.

Even with these contributions the present state

of the technology of high-temperature strain gages

leaves much to be desired. The major problems

result from inadequate properties in available materials. The lack of a suitable alloy (or other conductor) for the strain-sensing element is one of the

most serious obstacles. Karma and Evanohm,

which are modified 80 Ni-20 Cr alloys (table 1), have provided a partial solution for intermediate

temperatures, but above the 800 to 900 °F range,

the performance characteristics of these and other

alloys are severely limited. Evanohm and Karma

are metastable, solid-solution alloys which have

been given a special heat treatment so as to create a high electrical resistance and low-temperature

coefficient of resistance; even so, their gage factors

change with temperature and their drift rates in- crease with increasing temperature. Above about

900 °F most metals and alloys have such hightemperature coefficients of resistance that it is imperative to provide a means of correction; the

gage may be temperature compensated or if its temperature is known precisely a suitable correction can be made. In addition to large temperature coefficients, many of the otherwise useable

alloys undergo phase transformations on heating

and cooling, and these changes usually have

significant effects on the electrical resistance.

These and some other problems associated with the

selection of metals and alloys for high-temperature

strain gages have been discussed by Graft [15]

and Metcalfe [16]. The second major area in which additional

i improvement is needed is in the electrical insulai tion of cemented gages in the temperature region

above about 1,000 °F. There are two approaches

to the solution of this problem: One is to increase

the electrical resistance of the bonding cement,

and the other is to provide electrical insulation

which is independent of the cement, as for example,

by precoating the wire with a highly insulative

material. The first method would be generally more desirable, inasmuch as the gage could be

bonded and insulated in one operation. Cement

X-142 was developed to fill this need and, as indicated in table 2, in maintains comparatively

high resistivity; up to 70 times that of any of those cements which also maintain any measurable

bond strength. Yet, even a cement such as X-142 may still not have sufficient resistivity for

all types of measurement.

It is unlikely that cements chemically bonded

to a metal substrate could have high-temperature

resistivities comparable to the pure metal oxides

such as MgO, A12 03 , and Si02 . Many hightemperature cements contain such oxides as the

filler material, but the bonding agents which are added to impart the "cementing" property have

relatively low electrical resistivities. Furthermore, some ceramic cements react at high

temperatures with metal surfaces (see fig. 4) and

such reaction will result in large changes in

resistance of the strain-sensing element during

gage operation.

The technique of precoating a formed gage

with an insulating ceramic is one solution to these

problems. Both the protective quality of the

coating on a fine wire heated in ah and the insulative effect on a gage embedded in an incompletely

cured cement were demonstrated in the present

investigation. However, the high-temperature

resistivity of the vitreous-type coating that was

used was no better than that of the X-142

cement. Nevertheless, it is believed that with

the aid of the electrophoretic coating technique

as described in 3.4.b, ceramic coatings having

superior insulative qualities can be developed.

Such a coating might be composed almost entirely

of pure oxide grains held together by a network of a vitreous or sintered phase.

Inasmuch as the total thickness of the layer

between the gage elements and the substrate must

be kept to a minimum, it is finally concluded

that both the ceramic coating technique and the

best insulative cement should be utilized to the

maximum extent, if static strain measurements

free of electrical leakage errors are to be achieved

in the upper temperature regions.

6. References

[1] C. C. Perry and H. R. Lissner, The Strain gage primer,

p. 253, (McGraw-Hill Book Co., New York, N.Y.,

1955).

[2] Carl Schabtach and R. O. Fehr, Measurement of the damping of engineering materials during flexural vibrations at elevated temperatures, J. Appl.

Mechanics, ASME Trans. 66, A86 (1944).

19
[3] It. H. Kemp, W. C. Morgan, and S. S. Manson, The

application of high-temperature strain gages to the measurement of vibratory stresses in gas turbine

buckets, Nat. Advisory Comm. Aeronaut. Tech. Note 1174 (1947).

[4] F. G. Tatnall, Baldwin-Lima-Hamilton Corp., Phila- delphia 42, Pa., unpublished summary report on

high-temperature strain gage research, to Office of Naval Research, Washington 25, D.C., and Carbide

& Carbon Chemicals Co., Oak Ridge National

Laboratory, Oak Ridge, Tenn. (Jan. 25, 1955).

[5] R. H. Kemp, W. C. Morgan, and S. S. Manson,

Advances in high-temperature strain gages and their application to the measurements of vibratory stresses

in hollow turbine blades during engine operation, Proc. Soc. Exp. Stress Anal. [2] 8, 209 (1951).

[6] Anon., Properties of some metals and alloys, pp. 10-17,

(International Nickel Co., New York, N.Y., 1951).

[7] R. H. Kemp, Advances in static and dynamic hightemperature strain gage research, Symposium on

elevated temperature strain gages, Am. Soc. Testing

Materials, Spec. Tech. Publ. No. 230, p. 23 (1958).

[8] D. G. Moore, Bonding agents for high-temperature

strain gages, ibid, p. 106.

[9] J. R. Cuthill and W. N. Harrison, Effect of ceramic

coatings on the creep rate of metallic single crystal

and polycrystalline specimens, WADC Tech. Rept.

56-85 (1956).

[10] D. G. Moore and J. R. Cuthill, Protection of lowstrategic alloys with a chromium-boron-nickel cermet coating, Am. Ceram. Soc. Bull. [11] 34, 375

(1955).

[11] T. R. Walters, Terratex—a thin flexible inorganic

insulation, Trans. AIEE [1] 67, 123 (1948).

[12] Unpublished reports of the Engineering Mechanics

Section, NBS.

[13] J. W. Pitts, E. Buzzard, and D. G. Moore, Resistance measurement of ceramic-type strain gage cements,

Symposium on elevated-temperature strain gages, Am. Soc. Testing Materials, Spec. Tech. Publ. No. 230, p. 67 (1958).

[14] A. Wexler and S. Hasegawa, Relative humiditytemperature relationships of some saturated salt solutions in the temperature range 0° to 50° C,

J. Research NBS 53, 19 (1954) RP2512.

[15] W. H. Graft, Development of high-temperature strain gage wire, Symposium on elevated temperature

strain gages, Am. Soc. Testing Materials, Spec. Tech. Publ. No. 230, p. 76 (1958).

[16] A. G. Metcalfe, The Stability of strain gage alloy

wires, ibid, p. 109.

[17] E. W. Kammer and T. E. Pardue, Electrical resistance changes of fine wires during elastic and plastic

strains, Proc. Soc. Exp. Stress Anal. [1] 8, p. 7

(1949).

[18] Metals Handbook, Am. Soc. Metals, Cleveland 3, Ohio, pp. 1062, 1122-24 (1948).

Washington, D.C., September 15, 1960

20

U.S. GOVERNMENT PRINTING OFFICE : 1961
THE NATIONAL BUREAU OF STANDARDS

The scope of activities of the National Bureau of Standards at its major laboratories in Washington, D.C., and Boulder,

Colorado, is suggested in the following listing of the divisions and sections engaged in technical work. In general, each

section carries out specialized research, development, and engineering in the field indicated by its title. A brief description

of the activities, and of the resultant publications, appears on the inside of the front cover. WASHINGTON, D.C.

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